1. Field of the Invention
The present invention relates to a traction control system for motor vehicles, and more particularly it pertains to such a system wherein a slip of driving wheels which tends to be caused when the motor vehicle starts or accelerates is detected to control the braking of the driving wheels, thereby restraining the slip of the driving wheels.
2. Description of the Prior Art
Known in the art is a traction control system for motor vehicles, which is arranged, when the motor vehicle runs on a low-friction road surface such as a frozen, snow-covered or muddy one, to restrain a slip of driving wheel which tends to be caused in an attempt to start or accelerate the motor vehicle, thereby enhancing the starting/accelerating performance as well as the running stability of the vehicle. With such a conventional traction control system, it has been the usual practice that a slip of each of the left-hand and right-hand driving wheel is detected; and when the extent of the slip exceeds a predetermined value, a brake hydraulic pressure applied to a brake device for each driving wheel is increased so that the braking of the driving wheels is controlled, while at the same time the engine output is also controlled, as disclosed in Japanese Patent "Kokai" (Laid-opened) Publication No. 61-85248.
To control the brake hydraulic pressure to be applied to the driving wheels, the following procedures are commonly performed: A plurality of threshold levels are set up in relation to the vehicle speed so that a plurality of driving wheel speed ranges are established between adjacent ones of the aforementioned threshold levels; detection is made as to which speed range the current driving wheel speed belongs to, and detection is also made as to whether such driving wheel speed is increasing or decreasing; and buildup, reduction and holding of the brake hydraulic pressure for the driving wheels are controlled on the basis of such detections.
However, with the above-described traction control system, control of engine output is also effected in parallel with control of the brake fluid pressure so that when the control of the brake fluid pressure is effected on the basis of mere comparison of the driving wheel speed and the aforementioned speed range, excessive buildup/reduction of the brake fluid pressure is liable to occur, thus resulting in a hunting phenomenon tending to cause a great vibration to be generated in the vehicle body, which imparts an uncomfortable feeling to the passenger.
With a view to coping with such a situation, the inventors have proposed a traction control system for motor vehicles, which is adapted, during traction control, to prevent excessive buildup/reduction of the brake fluid pressure, thereby avoiding occurrence of vibration in the vehicle body (refer to Japanese Patent Application No. 63-10832).
In the traction control system as disclosed in the above Japanese patent application, a plurality of threshold values which are higher than the vehicle speed, are set up; during acceleration of the driving wheel, buildup of the brake fluid pressure is started at a time point when the speed of the driving wheel exceeds, for the first time, either a specific one or higher one of the plurality of threshold values; at a time point when a high peak of the driving wheel speed occurs where the driving wheel speed changes from increase to decrease, the buildup of the brake fluid pressure is interrupted, and the brake fluid pressure prevailing at the high-peak point is held; during deceleration of the driving wheel, at a time point when the driving wheel speed goes below any of the plurality of threshold values for the first time, reduction of the brake fluid pressure is started; and in the case where the reduction of the brake fluid pressure is continued until the driving wheel speed becomes lower than the lowest one of the plurality of threshold values, the reduction of the brake fluid pressure is interrupted at a time point when the driving wheel speed increases up to the lowest threshold value, and the brake fluid pressure prevailing at that time point is held. Furthermore, in the case where the driving wheel speed is in the region between the lowest and the highest threshold value of the plurality of threshold values, the reduction of the brake fluid pressure is interrupted at a time point when a low-peak of the driving wheel speed occurs where the driving wheel speed changes from decrease to increase, and the brake fluid pressure prevailing at the low-peak point is held.
As discussed above, with the above-described traction control system, control of the brake fluid pressure is effected in such a manner that buildup of the brake fluid pressure is interrupted at a time point when a high peak of the driving wheel speed as increasing is reached, and the brake fluid pressure prevailing at that time point is held; reduction of the brake fluid pressure is interrupted and changed to holding of the brake fluid pressure at a time point when a low peak of the driving wheel speed as decreasing occurs in the region between the lowest speed threshold value and the highest speed threshold value; and the holding of the brake fluid pressure is maintained until the driving wheel speed either exceeds a higher threshold value or goes below a lower threshold value. In this way, the brake fluid pressure is smoothly controlled without being excessively increased or reduced, thus preventing occurrence of vibration in the vehicle body which would otherwise tend to occur during traction control operation.
To achieve improved starting/accelerating performance while traction control is being effected, it is desirable that the brake fluid pressure be controlled in such a manner that the driving wheel speed when the motor vehicle starts or accelerates becomes closer not to the vehicle speed but to a speed which is higher than the vehicle speed by a predetermined slip ratio or slip quantity. With the system as disclosed in the above-mentioned Japanese Patent Application No. 63-10832, however, in the case where while the driving wheel speed is increasing gradually, a high peak of the driving wheel speed occurs so that the brake fluid pressure is held instead of being sufficiently increased, there is a tendency that such a situation persists that the driving wheel speed does not cross a higher threshold level. In such a case, the holding of the brake fluid pressure is continued so that the brake fluid pressure becomes short, and thus there is likelihood that with a much higher driving wheel speed than the target speed, engine torque and brake torque are balanced together such that excessive slip persists for a long time.